Animal models for glucocorticoid sensitive lymphosarcoma and estradiol, progesterone and prolactin sensitive mammary cancer are under active study. Recently, we have shown that glucocorticoid sensitive lymphosarcoma P1798 bears an electron microscopic anionic coat not present on the hormonally resistant strain. Studies are in progress on five tissue culture strains of varying sensitivity to glucocorticoid to see whether the anionic coat will vary in density as a function of hormonal sensitivity. Recent study with our mammary tumor systems involves the relationship between estradiol and prolactin receptor, and tumor regression after oophorectomy. Conditions have been found for reproducible conversion of an oophorectomy non-responsive tumor, containing estradiol receptor to an oophorectomy-responsive tumor. The mechanism of this conversion is a major target of the current study.